Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of “bio-integrated” technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in f...

Bio-Integrated Wearable Systems: A Comprehensive Review

Wearable or attachable health monitoring smart systems are considered to be the next generation of personal portable devices for remote medicine practices. Smart flexible sensing electronics are components crucial in endowing health monitoring systems with the capability of real-time tracking of physiological signals. These signals are closely associated with body conditions, such as heart rate, wrist pulse, body temperature, blood/intraocular pressure and blood/sweat bio-information. Monitoring such physiological signals provides a convenient and non-invasive way for disease diagnoses and health assessments. This Review summarizes the recent progress of flexible sensing electronics for their use in wearable/attachable health monitoring systems. Meanwhile, we present an overview of different materials and configurations for flexible sensors, including piezo-resistive, piezo-electrical, capacitive, and field effect transistor based devices, and analyze the working principles in monitoring physiological signals. In addition, the future perspectives of wearable healthcare systems and the technical demands on their commercialization are briefly discussed.

Flexible Sensing Electronics for Wearable/Attachable Health Monitoring

Healable, adhesive, wearable, and soft human-motion sensors for ultrasensitive human–machine interaction and healthcare monitoring are successfully assembled from conductive and human-friendly hybrid hydrogels with reliable self-healing capability and robust self-adhesiveness. The conductive, healable, and self-adhesive hybrid network hydrogels are prepared from the delicate conformal coating of conductive functionalized single-wall carbon nanotube (FSWCNT) networks by dynamic supramolecular cross-linking among FSWCNT, biocompatible polyvinyl alcohol, and polydopamine. They exhibit fast self-healing ability (within 2 s), high self-healing efficiency (99%), and robust adhesiveness, and can be assembled as healable, adhesive, and soft human-motion sensors with tunable conducting channels of pores for ions and framework for electrons for real time and accurate detection of both large-scale and tiny human activities (including bending and relaxing of fingers, walking, chewing, and pulse). Furthermore, the soft human-motion sensors can be enabled to wirelessly monitor the human activities by coupling to a wireless transmitter. Additionally, the in vitro cytotoxicity results suggest that the hydrogels show no cytotoxicity and can facilitate cell attachment and proliferation. Thus, the healable, adhesive, wearable, and soft human-motion sensors have promising potential in various wearable, wireless, and soft electronics for human–machine interfaces, human activity monitoring, personal healthcare diagnosis, and therapy.

Wearable, Healable, and Adhesive Epidermal Sensors Assembled from Mussel-Inspired Conductive Hybrid Hydrogel Framework

ZnO tetrapod materials for functional applications

Functional electrical devices have promising potentials in structural health monitoring system, human-friendly wearable interactive system, smart robotics, and even future multifunctional intelligent room. Here, a low-cost fabrication strategy to efficiently construct highly sensitive graphite-based strain sensors by pencil-trace drawn on flexible printing papers is reported. The strain sensors can be operated at only two batteries voltage of 3 V, and can be applied to variously monitoring microstructural changes and human motions with fast response/relaxation times of 110 ms, a high gauge factor (GF) of 536.6, and high stability >10 000 bending–unbending cycles. Through investigation of service behaviors of the sensors, it is found that the microcracks occur on the surface of the pencil-trace and have a major influence on the functions of the strain sensors. These performances of the strain sensor attain and even surpass the properties of recent strain sensing devices with subtle design of materials and device architectures. The pen-on-paper (PoP) approach may further develop portable, environmentally friendly, and economical lab-on-paper applications and offer a valuable method to fabricate other multifunctional devices.

Flexible and Highly Sensitive Strain Sensors Fabricated by Pencil Drawn for Wearable Monitor

Stretchable and multifunctional sensors can be applied in multifunctional sensing devices, safety forewarning equipment, and multiparametric sensing platforms. However, a stretchable and multifunctional sensor was hard to fabricate until now. Herein, a scalable and efficient fabrication strategy is adopted to yield a sensor consisting of ZnO nanowires and polyurethane fibers. The device integrates high stretchability (tolerable strain up to 150%) with three different sensing capabilities, i.e., strain, temperature, and UV. Typically achieved specifications for strain detection are a fast response time of 38 ms, a gauge factor of 15.2, and a high stability of >10 000 cyclic loading tests. Temperature is detected with a high temperature sensitivity of 39.3% °C−1, while UV monitoring features a large ON/OFF ratio of 158.2. With its fiber geometry, mechanical flexibility, and high stretchability, the sensor holds tremendous prospect for multiparametric sensing platforms, including wearable devices.

A Highly Stretchable ZnO@Fiber‐Based Multifunctional Nanosensor for Strain/Temperature/UV Detection

We report a self-powered strain sensor based on ZnO/PEDOT:PSS hybrid structure on a flexible polystyrene substrate. The electrical transport of the hybrid structure was modulated by the strain. The sensitivity of the fabricated device is enhanced to 1.0 × 104 under solar light, due to the piezo-phototronic effect of ZnO. The possible mechanism was discussed using energy band diagrams.

A self-powered strain senor based on a ZnO/PEDOT:PSS hybrid structure

Single crystalline zinc hydroxystannate (ZnSn(OH)6) nanocubes have been synthesized on α-{Cu,Sn} copper foils by a low-temperature hydrothermal method. The obtained ZnSn(OH)6 nanocubes have the cubic perovskite crystalline structure and uniform arrangement on the substrate. The possible growth process of the ZnSn(OH)6 crystalline is proposed. The well-dispersed distribution and uniform size of the ZnSn(OH)6 cubes are successfully achieved thanks to the substrate which plays an important role to control the nucleation site and provide Sn4+ ions. The size of the cubes can be adjusted from nanometre to micrometre scale by prolonging the reaction time. After the secondary nucleation, a novel morphology of mother–daughter double cubes was formed. The decomposition of ZnSn(OH)6 cubes annealed at high temperature is also investigated, and interesting hollow-shaped morphology changes are discovered. This method will be useful for the controllable fabrication of nanomaterials and potential applications in nanoscale devices.

Controllable synthesis of well-dispersed and uniform-sized single crystalline zinc hydroxystannate nanocubes

Single ZnO nanotetrapod-based sensors for monitoring localized UV irradiation were constructed with ohmic and Schottky contact characteristics. Localized UV irradiation at the third leg of the tetrapod was monitored by measuring the sensor's current response. Measurements of I–V performances and time-resolved current were conducted. The results demonstrate that localized UV irradiation can be detected in real time as electrical transport properties can be modulated by localized UV irradiation, and the higher the UV light power density gets, the larger the current response becomes, which is observed to be completely repeatable and reversible. Additionally, Schottky-contact type sensors clearly show a greater current response than ohmic-contact-type sensors, which further proved that Schottky-contact-type sensors are a better choice for detection in an irradiation environment. Two possible explanations are given for the phenomenon, including an electron transfer effect and a surface/interface effect on the band structure. The as-constructed sensors exhibit different sensitivities towards irradiation with various power densities, indicating that ZnO nanotetrapod-based sensors can be a promising candidate for detection in many areas including electron irradiation detection, ultraviolet irradiation monitoring, strain sensing, and complicated microenvironment observations such as biological cell inspection.

Single ZnO nanotetrapod-based sensors for monitoring localized UV irradiation

A novel electrophoresis based dye adsorption technique is developed for the process of ZnO-nanorod-array dye-sensitized solar cells (DSSC). By applying a constant current electrophoresis procedure, ZnO electrodes were successfully sensitized by N719 dye, and the effects of varying the electrophoretic current and duration time on the performance of DSSC were investigated. It is found that abundant dye adsorption can be obtained within significantly reduced sensitization time by electrophoresis compared to standard immersion method. Thicker ZnO-nanorod-array film and lower electrophoretic current requires longer electrophoresis time to reach the optimal sensitization effect, while very long sensitization time will result in dye agglomeration, which is harmful to the performance of DSSC. The fast dye adsorption process is more advantageous as the ZnO-nanorod-array film becomes thicker because dye molecules can readily penetrate into the depth of ZnO film in a very short time, during which the formation of Zn2+/dye complex can be suppressed.

Qinyu Wang

Papers

A Highly Stretchable ZnO@Fiber‐Based Multifunctional Nanosensor for Strain/Temperature/UV Detection

A self-powered strain senor based on a ZnO/PEDOT:PSS hybrid structure

Controllable synthesis of well-dispersed and uniform-sized single crystalline zinc hydroxystannate nanocubes

Single ZnO nanotetrapod-based sensors for monitoring localized UV irradiation

Fast sensitization process of ZnO-nanorod-array electrodes by electrophoresis for dye-sensitized solar cells